add translator for novel

lib/network/translator.dart

@@ -0,0 +1,75 @@
+import 'package:pixes/network/app_dio.dart';
+
+abstract class Translator {
+  static Translator? _instance;
+
+  static Translator get instance {
+    if (_instance == null) {
+      init();
+    }
+    return _instance!;
+  }
+
+  static void init() {
+    _instance = GoogleTranslator();
+  }
+
+  /// Translates the given [text] to the given [to] language.
+  Future<String> translate(String text, String to);
+}
+
+class GoogleTranslator implements Translator {
+  final Dio _dio = AppDio();
+
+  String get url => 'https://translate.google.com/translate_a/single';
+
+  Map<String, dynamic> buildBody(String text, String to) {
+    return {
+      'q': text,
+      'client': 'at',
+      'sl': 'auto',
+      'tl': to,
+      'dt': 't',
+      'ie': 'UTF-8',
+      'oe': 'UTF-8',
+      'dj': '1',
+    };
+  }
+
+  Future<String> translatePart(String part, String to) async {
+    final response = await _dio.post(
+      url,
+      data: buildBody(part, to),
+      options: Options(
+        headers: {
+          'Content-Type': 'application/x-www-form-urlencoded;charset=utf-8',
+        },
+      ),
+    );
+    var buffer = StringBuffer();
+    for(var e in response.data['sentences']) {
+      buffer.write(e['trans']);
+    }
+    return buffer.toString();
+  }
+
+  @override
+  Future<String> translate(String text, String to) async {
+    final lines = text.split('\n');
+    var buffer = StringBuffer();
+    var result = '';
+    for(int i=0; i<lines.length; i++) {
+      final line = lines[i];
+      if (buffer.length + line.length > 5000) {
+        result += await translatePart(buffer.toString(), to);
+        buffer.clear();
+      }
+      buffer.write(line);
+      buffer.write('\n');
+    }
+    if (buffer.isNotEmpty) {
+      result += await translatePart(buffer.toString(), to);
+    }
+    return result;
+  }
+}